1. Field of the Invention
The invention relates generally to Token ring networks including a plurality of stations serially connected by a transmission medium so as to form a closed loop on which packets of data are transferred, and more particularly to the transfer and storage of data within individual stations of such networks.
2. Description of the Related Art
A Token ring network comprises a collection of stations serially connected by a transmission medium so as to form a closed loop. The illustrative drawings of FIG. 1, depict a typical Token ring network. Information is transmitted sequentially, as a stream of symbols from one active station to the next. Inactive stations are bypassed. In the network of FIG. 1, information is serially transmitted from station S1 to S3 and from stations S3 through station Sn. Station S2 is bypassed. Station S2 is bypassed through the action of its bypass means b2. The transmission path of stations S1 through Sn can be reconfigured through the use of bypass means, b1 through bn.
In operation, each station regenerates and repeats symbols transmitted on the transmission medium and serves as a mechanism for attaching one or more devices to the network so that the attached devices can communicate with each other. In practice, for example, such attached devices can be computers and associated peripheral equipment.
Data circulated on the network are transmitted onto the transmission medium by the individual stations. The data typically circulate on the medium in packets. Each packet ordinarily includes a destination address indicating the address of the station to which the data packet is destined. In at least one Token ring network, when a data packet reaches its destination station that station copies the data and repeats the packet onto the medium. Finally, the station that originally transmitted the data packet onto the transmission medium removes it from circulation.
In a typical Token ring network, individual stations gain the right to transmit packets onto the medium by detecting a Token that circulates throughout the network. The Token is a control symbol. Any station, upon detecting the Token, may "capture" the Token and remove it from the transmission medium. Only a station that has captured the Token can transmit data packets to the transmission medium. After transmitting its data packets to the medium, a station transmits onto the network a new Token, which then may be captured by another station.
A Token-holding timer, or equivalent means, limits the length of time during which any individual station may use or occupy the medium before passing the Token.
The American National Standard For Information Systems has issued a proposed ANSI X3T9.5 Specification for Fiber-Distributed Data Interface (the FDDI Standard) which describes a media access control (MAC) protocol intended for use in high-performance multistation networks. The protocol is intended to be effective at 100 megabits per second using a Token Ring Architecture and Fiber Optics as the transmission medium.
The FDDI Standard establishes rules for the transmission of packets by individual stations to the transmission medium and for the reception of packets by individual stations from the transmission medium. For example, the Standard requires that a station transmit only complete packets, and not partial packets. If the allotted time during which a station can hold a Token expires during the transmission of a packet, the Standard requires the station to finish transmitting that packet before surrendering access to the medium and generating a new Token. Furthermore, for example, the Standard requires that each individual station extract packets from the transmission medium immediately upon their arrival at the station.
Thus, a Token ring network can have relatively stringent rules governing packet transfers between individual stations and the transmission medium. Such rules are important for efficient operation of the overall network.
Similarly, individual stations typically invoke rules for the efficient use of data packets while those packets reside at the station. These rules often involve schemes for efficiently storing data in memory so that they are readily available for use.
In order to comply with network rules governing data packet transfers to and from the transmission medium, and to also comply with individual station rules governing data storage, an effective method for transferring data between the transmission medium and memory of individual stations is needed. The present invention meets this need.